In U.S. Pat. No. 4,081,520, assigned to the assignee hereof, the disclosure of which is incorporated herein by reference, there is described the production of chlorine dioxide at high efficiency, i.e., greater than about 95% conversion, preferably at least about 98% conversion, of chlorate to chlorine dioxide, by the reaction of sodium chlorate with methanol in the presence of sulphuric acid in an aqueous reaction medium having a total acid normality of at least about 9 normal and which is maintained at its boiling point under a subatmospheric pressure. The process is operated continuously with chlorine dioxide being removed from the reaction zone in gaseous admixture with steam, aqueous sodium chlorate, methanol and sulphuric acid being fed to the reaction zone, and sodium acid sulphate precipitating in the reaction zone. This process is known as and referred to hereinafter as the "R8" process.
In U.S. Pat. No. 4,473,540, assigned to the assignee hereof, the disclosure of which is incorporated herein by reference, there is described a modification of the latter process wherein, under plant conditions, the total acid normality may range down to about 7 normal. In U.S. Pat. No. 4,465,658, assigned to the assignee hereof, the disclosure of which is incorporated herein by reference, there is described a further modification of the above-described process, wherein small quantities of chloride ions are continuously fed to the reaction medium in addition to the other reactants, so as to avoid the incidence of random periodic loss of chlorine dioxide production. The present invention also relates to a modification of the above-described R8 process.
It is also known from U.S. Pat. No. 2,863,722, assigned to the assignee hereof, the disclosure of which is incorporated herein by reference, to produce chlorine dioxide at high efficiency by reaction of sodium chlorate, sodium chloride and sulphuric acid at high acid normalities above about 5 normal. This process is known as and referred to hereinafter as the "R2" process. As described in Canadian Patent No. 825,084 to the assignee hereof, the disclosure of which is incorporated herein by reference, the reaction medium for the R2 process may be maintained at its boiling point while a subatmospheric pressure is applied to the reaction zone.
All processes for the generation of chlorine dioxide from sodium chlorate proceed in accordance with the equation: EQU ClO.sub.3.sup.- +Cl.sup.- +2H.sup.+ .fwdarw.Cl.sub.2 +1/2Cl.sub.2 +H.sub.2 O(1)
There is a competing reaction which produces only chlorine in accordance with the equation: EQU ClO.sub.3.sup.- +5Cl.sup.- +6H.sup.+ .fwdarw.3ClO.sub.2 +3H.sub.2 O(2)
The extent to which chlorate ions are reacted by equation (1) to produce chlorine dioxide is the efficiency of the process, which is expressed as a percentage.
In the R8 process, the chloride ions for the reaction of equations (1) and (2) are produced in situ by reduction of the co-produced chlorine by the methanol reactant. The gaseous product stream consists mainly of chlorine dioxide with small amounts of chlorine. In the R2 process, the chloride ions for the reactions of equations (1) and (2) are added from an external source in the form of sodium chloride. As a result, the gaseous product stream contains substantial quantities of chlorine.
Studies have determined that the R8 process is best described by the following equations: EQU 30NaClO.sub.3 +20H.sub.2 SO.sub.4 +12CH.sub.3 OH.fwdarw.30ClO.sub.2 +10Na.sub.3 H(SO.sub.4).sub.2 +23H.sub.2 O+5CH.sub.3 OH+6HCOOH+CO.sub.2( 3) EQU 12NaClO.sub.3 +8H.sub.2 SO.sub.4 +6CH.sub.3 OH.fwdarw.6ClO.sub.2 +3Cl.sub.2 +18H.sub.2 O+6CO.sub.2 +4Na.sub.3 H(SO.sub.4).sub.2 ( 4)
where equation (3) is the efficient process corresponding to equation (1) and equation (4) is the inefficient process corresponding to equation (2). A factor which promotes the efficient reaction of equation (3) is a concentration of sodium chlorate of above 1 molar, and efficiencies in excess of about 95%, usually at least about 97%, are attained.
Studies also have determined that the R2 process is best described by the following equations: EQU NaClO.sub.3 +NaCl+H.sub.2 SO.sub.4 .fwdarw.ClO.sub.2 +1/2Cl.sub.2 +H.sub.2 O+Na.sub.2 SO.sub.4 ( 5) EQU NaClO.sub.3 +5NaCl+3H.sub.2 SO.sub.4 .fwdarw.3Cl.sub.2 +3H.sub.2 O+3Na.sub.2 SO.sub.4 ( 6)
where equation (5) is the efficient process corresponding to equation (1) and equation (6) is the inefficient process corresponding to equation (2). This process usually operates at a chlorate concentration of about 0.2 molar in the reaction medium to maintain the production rate under control. The kinetics of chlorine dioxide production by this process is proportional to [H.sup.+ ].sup.4 [ClO.sub.3.sup.- ].sup.2 [Cl.sup.- ].sup.2.
Sodium chlorate for reaction to form chlorine dioxide is formed by electrolysis of sodium chloride solution. In such an electrolysis process, not all the sodium chloride feed is electrolyzed and hence the sodium chlorate solution which results contains unreacted sodium chloride. This solution of sodium chlorate and sodium chloride is commonly termed "cell liquor" and the latter term is used herein to refer to such solution. For use in the conventional R8 process, the sodium chlorate is crystallized from the electrolysis product solution and then redissolved in water to provide the required concentration of sodium chlorate feed solution. For use in the R2 process, a similar crystallization and redissolution may be effected or the cell liquor may be supplemented with sodium chloride to the required feed concentration.
In the marketplace, sodium chlorate is available not only in the form of sodium chlorate crystals but also as cell liquor. In addition, in some pulp mills, cell liquor is produced on site. To increase the flexibility of choice of raw material supplies it often is desirable to use cell liquor directly in the production of chlorine dioxide.
It has previously been thought, however, that it was not possible to use cell liquor as a feed to the R8 process for the following reasons. Firstly, as noted earlier, a concentration of at least 1 molar sodium chlorate is desirable in the reaction medium to promote efficient chlorine dioxide production by the R8 process. This requirement is not compatible with maintaining about 0.2M chlorate ion for control over the production of chlorine dioxide by the R2 process. Secondly, cell liquor usually contains sodium dichromate, added to the sodium chloride solution to assist in the efficiency of electrolysis, and in the presence of reducing agents, such as methanol and chloride ions, Cr.sup.VI is rapidly reduced to Cr.sup.III. It is known from an article entitled "ERCO R7 ClO.sub.2 Generator Helps Balance Pulp Mill Chemical Supply" by M. C. J. Fredette (one of the inventors herein), reported in Pulp & Paper Canada 85:1 (1984) pp T17 and 18, that the presence of Cr.sup.III in the chlorine dioxide generating process described in U.S. Pat. No. 4,086,329, assigned to the assignee hereof, the disclosure of which is incorporated herein by reference, causes major disturbances to chlorine dioxide generation. The Cr.sup.III, at levels of 200 to 300 ppm incorporated in the salt cake, acts as a nucleating site for both neutral and acidic sodium sulphates and very fine, difficult to filter, crystals are produced which thicken the reaction medium and prevent normal chlorine dioxide gas release.
It was thought, therefore, that, if typical cell liquor containing 600 gpl NaClO.sub.3 and 100 gpl NaCl were attempted to be used as feed for the R8 chlorine dioxide generator, then the resulting chlorine dioxide generating process, wherein approximately 70% of the chlorate ions would react with methanol by the R8 process and approximately 30% of the chlorate ions would react with chloride ions by the R2 process, there would result an uncontrolled process, beset by the problems of the presence of Cr.sup.III if the cell liquor contained sodium dichromate.